1. Field of the Invention
The present invention relates to a burner for producing a porous glass preform, having a multi-nozzle type concentric multi-tube structure in which multiple gas jet ports are disposed concentrically.
2. Description of Related Art
Conventionally, various methods have been proposed to produce an optical fiber preform. Among them, the outside vapor deposition method (OVD method) is generally used since the refractive-index distribution of an optical fiber preform to be obtained can be set relatively freely, and moreover optical fiber preforms with large diameters can be mass-produced. The OVD method includes depositing and attaching glass particulates generated in a flame of a burner onto a rotating starting member by relatively reciprocating the burner or the starting member, so as to synthesize a glass particulate deposition (hereinafter, referred to as “soot”), and dehydrating and sintering the resultant soot in an electric furnace to produce a transparent glass body.
Conventionally, a concentric multi-tube burner has been used for synthesizing the soot. Such a structure of the burner does not allow glass material gas, combustible gas, and combustion-supporting gas to be mixed well, so that glass particulates are not produced sufficiently. As a result, the yield of the glass particulates is not satisfactory, thereby causing difficulties in synthesizing them at high speed.
To solve the above problem, Japanese Patent (JP-B) No. 1773359 proposes a multi-nozzle type burner in which small-diameter combustion-supporting gas jet ports are disposed in a combustible gas jet port to surround a material gas jet port arranged in a center of the burner.
Moreover, the following methods have been proposed to further improve deposition efficiency with this type of burner.
Japanese Patent Application Publication (JP-A) No. 2003-206154, JP-A No. 2004-331440, JP-A No. 2006-182624, and JP-B No. 3744350, for example, propose configurations of the small-diameter combustion-supporting gas jet ports. Meanwhile, JP-A No. Hei 05-323130, JP-B No. 3543537, and JP-A No. 2003-226544, for example, propose optimization of focal lengths of the small-diameter combustion-supporting gas jet ports. In addition, Japanese Patent No. 3591330, JP-A No. 2003-165737, JP-A No. 2003-212555, and Japanese Patent No. 3653902, for example, propose optimization of a gas flow rate and a gas linear velocity.
The burner for producing a porous glass preform is generally made of silica glass to prevent contamination, and is manually fabricated by a glass blower. In particular, the multi-nozzle type burner has a more complex structure than a conventional concentric multi-tube burner and, specifically, it is highly difficult to fabricate the multi-nozzle type burner having a configuration in which multiple gas jet ports are arranged outside the small-diameter gas jet port and are reduced in their diameters at the tip side.
Therefore, although the multi-nozzle type burner having such a complex structure is fabricated by a skilled glass blower with satisfactory attention to precision, individual differences of the burners tend to be larger than a conventional concentric multi-tube burner. Such individual differences of the burners result from tolerance stack-up of members.
The individual differences disadvantageously cause variations in deposition efficiency even under the same deposition conditions or result in damage of the burner tip by being burned.